/**
 * @author Pacien TRAN-GIRARD
 * @author Timothée FLOURE
 */
public final class Filter {

    public static final float[][] SMOOTH_CORE = new float[][]{
            {0.1f, 0.1f, 0.1f},
            {0.1f, 0.2f, 0.1f},
            {0.1f, 0.1f, 0.1f}
    };
    public static final float[][] SOBEL_X_CORE = new float[][]{
            {-1, 0, 1},
            {-2, 0, 2},
            {-1, 0, 1}
    };
    public static final float[][] SOBEL_Y_CORE = new float[][]{
            {-1, -2, -1},
            {0, 0, 0},
            {1, 2, 1}
    };


    /**
     * Get a pixel without accessing out of bounds
     *
     * @param gray a HxW float array
     * @param row  Y coordinate
     * @param col  X coordinate
     * @return nearest valid pixel color
     */
    public static float at(float[][] gray, int row, int col) {
        int maxRow = gray.length - 1;
        int maxCol = gray[0].length - 1;

        if (row < 0) row = 0;
        if (col < 0) col = 0;

        if (row > maxRow) row = maxRow;
        if (col > maxCol) col = maxCol;

        return gray[row][col];
    }

    /**
     * Convolve a single-channel image with specified kernel.
     *
     * @param gray   a HxW float array
     * @param kernel a MxN float array, with M and N odd
     * @return a HxW float array
     */
    public static float[][] filter(float[][] gray, float[][] kernel) {
        int width = gray[0].length;
        int height = gray.length;
        float[][] filteredImage = new float[height][width];
        for (int row = 0; row < height; ++row) {
            for (int col = 0; col < width; ++col) {
                float pixelNeighbors[][] = {
                        {Filter.at(gray, row - 1, col - 1), Filter.at(gray, row - 1, col), Filter.at(gray, row - 1, col + 1)},
                        {Filter.at(gray, row, col - 1), Filter.at(gray, row, col), Filter.at(gray, row, col + 1)},
                        {Filter.at(gray, row + 1, col - 1), Filter.at(gray, row + 1, col), Filter.at(gray, row + 1, col + 1)}
                };

                for (int i = 0; i < kernel[0].length; i++) {
                    for (int j = 0; j < kernel.length; j++) {
                        filteredImage[row][col] += kernel[i][j] * pixelNeighbors[i][j];
                    }
                }
            }
        }

        return filteredImage;
    }

    /**
     * Smooth a single-channel image
     *
     * @param gray a HxW float array
     * @return a HxW float array
     */
    public static float[][] smooth(float[][] gray) {
        float[][] smoothtImage = Filter.filter(gray, SMOOTH_CORE);
        return smoothtImage;
    }

    /**
     * Compute horizontal Sobel filter
     *
     * @param gray a HxW float array
     * @return a HxW float array
     */
    public static float[][] sobelX(float[][] gray) {
        float[][] sobelXImage = Filter.filter(gray, SOBEL_X_CORE);
        return sobelXImage;
    }

    /**
     * Compute vertical Sobel filter
     *
     * @param gray a HxW float array
     * @return a HxW float array
     */
    public static float[][] sobelY(float[][] gray) {
        float[][] sobelYImage = Filter.filter(gray, SOBEL_Y_CORE);
        return sobelYImage;
    }

    /**
     * Compute the magnitude of combined Sobel filters
     *
     * @param gray a HxW float array
     * @return a HxW float array
     */
    public static float[][] sobel(float[][] gray) {
        float[][] x = Filter.sobelX(gray);
        float[][] y = Filter.sobelY(gray);

        int width = gray[0].length;
        int height = gray.length;

        float[][] sobelImage = new float[height][width];

        for (int row = 0; row < height; ++row) {
            for (int col = 0; col < width; ++col) {
                sobelImage[row][col] = (float) Math.sqrt(Math.pow(x[row][col], 2) + Math.pow(y[row][col], 2));
            }
        }

        return sobelImage;
    }

}